Comment by adamcharnock

I would highlight a distinction between Queues and Streams, as I think this is an important factor in making this choice.

In the case of a queue, you put an item in the queue, and then something removes it later. There is a single flow of items. They are put in. They are taken out.

In the case of a stream, you put an item in the queue, then it can be removed multiple times by any other process that cares to do so. This may be called 'fan out'.

This is an important distinction and really effects how one designs software that uses these systems. Queues work just fine for, say, background jobs. A user signs up, and you put a task in the 'send_registration_email' queue.[1]

However, what if some _other_ system then cares about user sign ups? Well, you have to add another queue, and the user sign-up code needs to be aware of it. For example, a 'add_user_to_crm' queue.

The result here is that choosing a queue early on leads to a tight-coupling of services down the road.

The alternative is to choose streams. In this case, instead of saying what _should_ happen, you say what _did_ happen (past tense). Here you replace 'send_registration_email' and 'add_user_to_crm' with a single stream called 'used_registered'. Each service that cares about this fact is then free to subscribe to that steam and get its own copy of the events (it does so via a 'consumer group', or something of a similar name).

This results in a more loosely coupled system, where you potentially also have access to an event history should you need it (if you configure your broker to keep the events around).

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This is where Postgresql and SQS tend to fall down. I've yet to hear of an implementation of streams in Postgresql[2]. And SQS is inherently a queue.

I therefore normally reach for Redis Steams, but mostly because it is what I am familiar with.

Note: This line of thinking leads into Domain Driven Design, CQRS, and Event Sourcing. Each of which is interesting and certainly has useful things to offer, although I would advise against simply consuming any of them wholesale.

[1] Although this is my go-to example, I'm actually unconvinced that email sending should be done via a queue. Email is just a sequence of queues anyway.

[2] If you know of one please tell me!